The recovery of oil from oil fields often requires injection of a displacing fluid, most often water, to maintain the pressure in the reservoir so as to allow production, through displacement of the oil in place, from injection wells to production wells arranged according to a previously optimized scheme for the field considered.
In the case of water injection, this displacing fluid can be injected alone or it can, on the contrary, contain chemical agents intended to improve sweeping of the oil in place.
Among these chemical agents, on the one hand, surfactants are intended to reduce trapping of the oil in the pores of the rock through reduction of the water-oil interfacial tension and possibly modification of the rock wettability; on the other hand, polymers provide higher viscosity to the water, thus increasing its hydrocarbon phase sweep efficiency.
However, these two categories of enhancing products undergo losses in the reservoir due to many phenomena, among which retention or adsorption of the products on the rock, which can be high and obviously detrimental to the economic interest of such recovery methods. The presence of divalent cations in place in the reservoir water and on the rock minerals (notably clays) still increases these losses.
The injection of other agents, generally alkaline products (such as sodium carbonate, soda, etc.) is therefore recommended prior to injecting aqueous sweeping fluids containing enhancing products such as surfactants and polymers. These rock conditioning agents, dissolved in various chemical forms, dissociated or not into ions, involve many chemical equilibria in aqueous phase:                salt precipitation reactions (divalent cation salts in place notably),        multiple interactions with the rock (ion exchange and adsorption with modification of the charges of the solid surface),        possibly also reactions with some constituents of the oil in place (formation of soaps with the surfactant). The injection of a conditioning agent alone can even already improve the recovery of oil in relation to the conventional injection of water without any chemical agent.        
All these physico-chemical phenomena have to be taken into account in order to determine the volumes and concentrations of the products to be injected, and the modes of injection (flow rates, distribution in the field via the injection scheme, etc.), for the phase of conditioning the reservoir rock (injection of alkaline conditioning agents) as well as, subsequently, the enhanced water slugs (through surfactants and/or polymers), intended to improve the recovery and the displacement efficiency.
This dimensioning of the injected solutions is essential because it determines the feasibility and the profitability of these methods, via:
(a) the size and the cost of the facilities: surface facilities for preparing the solutions (surfactants, polymers); number, arrangement and well pumping equipments,
(b) the conditioning product and enhancer masses required (volumes and concentrations), therefore their cost,
(c) and, of course, the efficiency in terms of oil recovery.
Dimensioning these conditioning product and enhancer injections involves computations on a reservoir model discretized in form of elementary units of volume (cells), wherein the fluxes of the phases in presence (water and hydrocarbon phases: oil and/or gas), the transport and the evolution of the chemical species (bringing into or keeping in solution, precipitation, adsorption, conversion to other chemical species) have to be calculated so as to determine the amounts of product lost within the reservoir and the oil recovered in the production wells. This dimensioning involves studying the sensitivity to the multiple operating parameters (concentrations, slug size, flow rates, well placement, etc.), which therefore requires a reliable, powerful (fast) and efficient (in terms of usability of the results) simulator.
Reliability implies taking account of the various physico-chemical mechanisms involved. Power means short simulation times so as to be able to simulate multiple scenarios intended to understand and to select a dimensioning that guarantees the feasibility and maximizes the profitability of the operation.
Finally, the simulator efficiency means here a prediction tool requiring known information in a number of data as limited as possible by the user, and whose results interpretation and optimization is easy and fast via sensitivity studies with a small number of input parameters.
Considering the complex mode of action of the aforementioned products, the prediction models are complex because they usually include the multiple chemical species involved in the equilibrium reactions within the aqueous phase, such as: water-oil equilibrium, reactions of precipitation, adsorption on the rock, or others.
Taking into account all the chemical species and all the phenomena involved in the transport of the enhancing and conditioning products on the reservoir scale considerably increases the size of the numerical systems to be solved and therefore the computation time.